CS444/CS544 Operating Systems Finale 4/27/2007 Prof. Searleman

Outline Return & discuss Exam#2 Final Exam: Thursday, May 3 rd, 7:00 pm Distributed Systems – a primer Office hours during finals week: posted on my office door

Grading Policy 30% Midterm Exams 30% Final Exam One required section Optional questions (for improving your grade) Bring a calculator if you plan to do the optional questions Bring your old exams & turn them in with the final 10% Homework will drop the lowest 10 point HW 30% Laboratory Projects Lab grades will be posted in your AFS subdirectory Second Chance lab: subdirectory 2ndchance/ make all changes by Wednesday

search TLB fetch PTE page fault overwrite victim page copy victim to disk read in new page fetch word hit miss valid invalid empty frame or clean victim victim page is dirty

A Distributed System

Loosely Coupled Distributed Systems Users are aware of multiplicity of machines. Access to resources of various machines is done explicitly by: Remote logging into the appropriate remote machine. Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism.

Tightly Coupled Distributed-Systems Users not aware of multiplicity of machines. Access to remote resources similar to access to local resources Examples Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task. Computation Migration – transfer the computation, rather than the data, across the system.

Distributed Operating Systems Process Migration – execute an entire process, or parts of it, at different sites. Load balancing – distribute processes across network to even the workload. Computation speedup – subprocesses can run concurrently on different sites. Hardware preference – process execution may require specialized processor. Software preference – required software may be available at only a particular site. Data access – run process remotely, rather than transfer all data locally.

Why Distributed Systems? Communication Resource sharing Computational speedup Reliability

Resource Sharing Distributed Systems offer access to specialized resources of many systems Example: Some nodes may have special databases Some nodes may have access to special hardware devices (e.g. tape drives, printers, etc.) DS offers benefits of locating processing near data or sharing special devices

OS Support for resource sharing Resource Management? Distributed OS can manage diverse resources of nodes in system Make resources visible on all nodes Like VM, can provide functional illusion bur rarely hide the performance cost Scheduling? Distributed OS could schedule processes to run near the needed resources If need to access data in a large database may be easier to ship code there and results back than to request data be shipped to code

Design Issues Transparency – the distributed system should appear as a conventional, centralized system to the user. Fault tolerance – the distributed system should continue to function in the face of failure. Scalability – as demands increase, the system should easily accept the addition of new resources to accommodate the increased demand. Clusters vs Client/Server Clusters: a collection of semi-autonomous machines that acts as a single system.

Why Distributed Systems? Communication Resource sharing Computational speedup Reliability

Computation Speedup Some tasks too large for even the fastest single computer Real time weather/climate modeling, human genome project, fluid turbulence modeling, ocean circulation modeling, etc. What to do? Leave the problem unsolved? Engineer a bigger/faster computer? Harness resources of many smaller (commodity?) machines in a distributed system?

Breaking up the problems To harness computational speedup must first break up the big problem into many smaller problems More art than science? Sometimes break up by function Pipeline? Job queue? Sometimes break up by data Each node responsible for portion of data set?

Linear Speedup Linear speedup is often the goal. Allocate N nodes to the job goes N times as fast Once you’ve broken up the problem into N pieces, can you expect it to go N times as fast? Are the pieces equal? Is there a piece of the work that cannot be broken up (inherently sequential?) Synchronization and communication overhead between pieces?

OS Support for Parallel Jobs Process Management? OS could manage all pieces of a parallel job as one unit Allow all pieces to be created, managed, destroyed at a single command line Fork (process,machine)? Scheduling? Programmer could specify where pieces should run and or OS could decide Process Migration? Load Balancing? Try to schedule piece together so can communicate effectively

OS Support for Parallel Jobs (con’t) Group Communication? OS could provide facilities for pieces of a single job to communicate easily Location independent addressing? Shared memory? Distributed file system? Synchronization? Support for mutually exclusive access to data across multiple machines Can’t rely on HW atomic operations any more Deadlock management? We’ll talk about clock synchronization and two-phase commit later

Why Distributed Systems? Communication Resource sharing Computational speedup Reliability

Distributed system offers potential for increased reliability If one part of system fails, rest could take over Redundancy, fail-over !BUT! Often the reality is that distributed systems offer less reliability “A distributed system is one in which some machine I’ve never heard of fails and I can’t do work!” Hard to get rid of all hidden dependencies No clean failure model Nodes don’t just fail they can continue in a broken state Partition network = many many nodes fail at once! (Determine who you can still talk to; Are you cut off or are they?) Network goes down and up and down again!

Robustness Detect and recover from site failure, function transfer, reintegrate failed site Failure detection Detecting hardware failure is difficult. To detect a link failure, a handshaking protocol can be used. Reconfiguration When Site A determines a failure has occurred, it must reconfigure the system: When the link or the site becomes available again, this information must again be broadcast to all other sites.

Issues for Distributed Systems Components can fail (not fail-stop) Network partitions can occur in which each portion of the distributed system thinks they are the only ones alive Don’t have a shared clock Can’t rely on hardware primitives like test-and- set for mutual exclusion …

Distributed Coordination To tackle this complexity need to build distributed algorithms for: Event Ordering Mutual Exclusion Atomicity Deadlock Handling Election Algorithms Reaching Agreement

In Summary... Course Objectives To introduce students to the abstractions and facilities provided by modern operating systems. To familiarize students with the issues that arise when implementing operation system services

In Summary... Course Objectives To introduce students to the abstractions and facilities provided by modern operating systems. To familiarize students with the issues that arise when implementing operation system services An Operating System is a software layer that... manages resources (“Who gets what, when”) provides an abstraction of the underlying hardware that is easier to program and use (“Virtual Machine”) Applications Operating Systems Hardware

This Semester Architectural support for OS; Application demand on OS Major components of an OS: Scheduling, Memory Management, Synchronization, File Systems,... How is the OS structured internally and what interfaces does it provide for using its services and tuning its behavior? What are the major abstractions modern OSes provide to applications and how are they supported?